This invention generally relates to saw chain drive devices, and more specifically, to devices of this type comprising a clutch drum and a drive means permanently fixed on the clutch drum.
The invention relates to two forms of drive means, a fixed drive sprocket which directly drives the saw chain, and a fixed drive spline which drives the saw chain indirectly with the addition of a replaceable drive sprocket. In the first instance the drive sprocket is permanently fixed to the clutch drum and in the second instance the drive spline is permanently fixed to the clutch drum.
It is conventional in the manufacture of saw chain drive devices to fix the drive means to the clutch drum by brazing. The weakness of this construction is that the torque transferred from the clutch drum to the drive means is limited by the strength and the quality of the brazed joint and the brazing material.
A basic improvement on this conventional manufacturing process is disclosed in Australian Patent Application No. 895,636 and PCT Patent Application No. PCT/Au90/00320. In accordance with this improvement, there is provided a saw chain drive device comprising a clutch drum and drive means, said clutch drum comprising an end wall provided with at least two apertures, with at least one of said apertures being radially spaced from the axis of rotation of said saw chain drive device, said drive means being provided with at least two projections located so as to be received in said apertures of said end wall such that said drive means is rotationally immovable relative to said clutch drum.
The above-identified Australian and PCT patent applications also disclose a method of forming a saw chain drive device comprising a clutch drum and drive means, said drive means being provided with at least two projections, said projections being used as a punch tool to form at least one aperture radially spaced from the axis of rotation within the end wall of the said clutch drum.
This improved saw chain drive device relies on a mechanical connection between the drive means and the clutch drum, and can be used as a substitute or in addition to conventional brazing techniques to give added strength to the final product.
This improved product has very high torsional strength; however, the above-mentioned mechanical connection between the drive means and the clutch drum does not provide for any lateral fixing of the drive means to the clutch drum--that is, along the axis of the chain drive device. As a result of this, the drive means has a tendency to work itself loose from the clutch drum.